Method of providing a fluorescent lamp stem with an integral mercury-vapor pressure regulating means

ABSTRACT

The mercury-vapor pressure within a fluorescent lamp is controlled by a metal (indium or an alloy of indium and tin, for example) that wets glass, combines with the mercury within the lamp to form an amalgam and is divided into segments which are fused directly to one or both of the glass stems at spaced locations. The amalgamative-metal segments are of such shape and mass that they inherently remain in place on the stem when they are heat-softened and pressed onto the glass surface. Segments of larger size and mass are retained in place by an overlying porous layer of inert material that adheres to the glass. The amalgamative metal can also be combined with a fusible binder to form a composite which is divided into small pellets that are pressed onto the glass stem and held in place by the adhesive action of the binder when the latter is fused during the bulblehring operation required to fabricate the lamp.

United States Patent [191 Morehead Aug. 12, 1975 [75] Inventor: Chalmers Morehead, Upper Montclair, NJ.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: May 28, 1974 [21] Appl. No.: 473,959

Related US. Application Data [62] Division of Ser. No. 293,239, Sept. 28, 1972.

[52] US. Cl. 29/25.l3; 29/25.1l; 313/178; 313/490; 316/25 [51] Int. Cl. HOlj 9/18; HOlj 7/18; HOlj 7/26 [58] Field of Search 313/178, 174, 229, 490, 313/225; 29/251], 25.13; 316/3, 12, 25; 1 17/335 L [56] References Cited UNITED STATES PATENTS 275,613 4/1883 Edison 316/3 X 2,262,177 11/1941 Gerrner... 313/225 X 2,280,618 4/1942 Besson 313/225 X 3,152,278 10/1964 Dziergwa et a1. 313/174 3,287,587 11/1966 Menelly 313/178 X 3,521,110 7/1970 Johnson 313/229 X 3,549,937 12/1970 Manada et al. 3,629,641 12/1971 Hofmann 313/178 X Primary ExaminerRoy Lake Assistant Examiner-James W. Davie Attorney, Agent, or FirmD. S. Buleza ABSTRACT The mercury-vapor pressure within a fluorescent lamp is controlled by a metal (indium or an alloy of indium and tin, for example) that wets glass, combines with the mercury within the lamp to form an amalgam and is divided into segments which are fused directly to one or both of the glass stems at spaced locations. The

amalgamative-metal segments are of such shape and mass that they inherently remain in place on the stern when they are heat-softened and pressed onto the glass surface. Segments of larger size and mass are retained in place by an overlying porous layer of inert material that adheres to the glass. The amalgamative metal can also be combined with a fusible binder to form a composite which is divided into small pellets that are pressed onto the glass stem and held in place by the adhesive action of the binder when the latter is fused during the bulb-lehring operation required to fabricate the lamp.

9 Claims, 8 Drawing Figures METHOD OF PROVIDING A FLUORESCENT LAMP STEM WITH AN INTEGRAL MERCURY-VAPOR PRESSURE REGULATING MEANS CROSS-REFERENCES TO RELATED APPLICATIONS This application is a division of application Ser. No. 293,239 filed Sept. 28, 1972.

BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to low-pressure electric discharge lamps and has particular reference to a method .of providing a fluorescent lamp stem with an integral means for controlling the mercury-vapor pressure within the finished lamp.

2. Description of the Prior Art Low-pressure mercury-vapor discharge lamps that contain a strategically located metal (such as indium, cadmium, etc.) which forms an amalgam with mercury and regulates the mercury-vapor pressure within the lamp during operation are well known in the art. A fluorescent lamp of this type having an amalgam-forming metal located on the inner surface of the phosphorcoated envelope is described in US. Pat. No. 3,007,071 issued Oct. 31, I961 to A. Lompe et al.

It is also known in the art that the time required for a fluorescent lamp to reach full light output from a cold start" can be reduced by placing an auxiliary quantity of amalgam at a location within the lamp where it will be quickly heated and thus rapidly supply mercury-vapor to the discharge. A lamp having such an auxiliary amalgam disposed on one of the anodes near the cathode is disclosed in US. Pat. No. 3,227,907 issued .Ian. 4, 1966 to C. I. Bernier et al.

A high-output fluorescent lamp having a continuous coating of amalgamative metal on the tubular portions of the glass stems is disclosed in US. Pat. No. 3,287,587 issued Nov. 22, 1966 to R. A. Menelly.

A fluorescent lamp containing an amalgamative metal that is divided into two flat strips which are held in place on the stem by a wire-mesh collar assembly is disclosed in U.S. Pat. No. 3,534,212 issued Oct. 13, I970 to G. S. Evans. An improved lamp of this type in which the amalgamative-metal strips are retained in place on the stern within the wire-mesh collar assembly by an overlying strip of fine-wire mesh is disclosed in US. Pat. No. 3,422,299 issued Jan. 14, 1969 to C. Morehead, the author of the present invention.

While the aforesaid prior art innovations achieved the desired primary objective of controlling the mercury-vapor pressure within the lamp when the latter is operated under high ambient-temperature conditions or at high power loadings, they are not entirely satisfactory from a cost or manufacturing standpoint since they require relatively expensive components or critical time-consuming assembly operations.

Applying the amalgamative metal to the stern in the form of a continuous coating poses another problem in that it limits the quantity of metal that can be placed at the proper location within the lamp. Larger stems or thick coatings must therefore be used in lamps of long length that require large quantities of amalgam. Thick coatings are impractical since they permit the fluid amalgam to drip from the stem and thus become dislocated.

There is, accordingly, a need for an improved method of making lamps of the amalgam type that per mits the lamp components to be fabricated and assembled on an efficient mass-production basis.

SUMMARY OF THE INVENTION The aforesaid objective is achieved in accordance with the present invention by dividing the amalgamforming metal into a number of small segments or *bits" and placing them at spaced locations directly on the glass stem a predetermined distance from the associated electrode. According to one embodiment, the bits of amalgam-forming metal are of such size and shape that they are held in place on the glass stem solely by the fact that the metal wets and thus inherently sticks to the glass surface. In other embodiments, the segments of amalgam-forming metal are of larger size and are held in place by a porous adherent coating of inert material or by an admixed porous binder.

The flared or tubular portions of the glass stems can also be provided with spaced indents or groove-like recesses that serve as retaining pockets for the amalgamative metal bits.

Since the present invention permits a large total quantity of amalgamative metal to be placed and retained directly on the glass stem without any separate holding means (or with the aid ofa simple and inexpensive retaining component), it reduces the manufacturing cost of the lamp and permits amalgam-type vaporpressure control means to be employed in standard 40 watt fluorescent lamps having short stems as well as in higher wattage lamps that have stems of longer length.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the invention will be obtained from the exemplary embodiments that will be subsequently described and are illustrated in the accompanying drawings, in which:

FIG. I is an elevational view of a 40 watt fluorescent lamp having a glass stem made in accordance with the invention, portions of the envelope being broken away for illustrative purposes;

FIG. 2 is an enlarged perspective view of one of the amalgam-bearing stems employed in the lamp shown in FIG. I;

FIG. 3 is a cross-sectional view of the amalgamcarrying portion of the stem taken along the line lII--III of FIG. 2;

FIG. 4 is a pictorial view of an alternative embodiment wherein the segments of the amalgamative metal are retained in place on the stem by an overlying porous layer of inert material, a part of which is removed for illustrative purposes.

FIG. 5 is an enlarged cross-sectional view of the amalgam-carrying portion of the stem and the associatcd coating along the line V-V of FIG. 4;

FIG. 6 is a perspective view of another lamp stern and amalgam structure made in accordance with an alternative emhodiment of the invention;

FIG. 7 is an enlarged cross-sectional view of the amalgam-carrying portion of the stem taken along the line VII-VII of FIG. 6; and

FIG. 8 is a similar view of another embodiment wherein the amalgamative metal is disposed in spaced indents or cavities formed in the outer surface of the stern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With specific reference now to the drawings, in FIG. I there is shown a fluorescent lamp L having the usual tubular vitreous envelope 12 the inner surface of which is coated with a layer 13 of a suitable ultravioletresponsive phosphor. The envelope 12 is closed at each end by a vitreous stem S that consists of a glass tube 14 having a flared end portion 15 that is fused to the rim of the envelope. A thermionic electrode 16 is supported at each end of the envelope I2 by lead wires 17 and 18 that are embedded in the fused press-sealed end of the stem tube 14 and extend through suitable base members 20 attached to the ends of the envelope and, thence, into hollow pin terminals 22 to which they are electrically connected, as by welding. After the lamp has been baked and evacuated it is charged with a suitable inert fill gas (argon or a mixture of argon and neon at about 3 mm. of Hg. pressure, for example), dosed with a measured amount of mercury and tipped-off in the well known manner.

In accordance with the present invention, the mercury-vapor pressure within the lamp L during operation is controlled by a plurality of discrete segments or bits 24 of a suitable amalgam-forming metal (such as indium or an indium-rich alloy of indium and tin) that are attached directly to one or both of the stems S at spaced locations around the circumference of the stem or stems a predetermined distance from the associated electrodes 16.

As shown more particularly in FIGS. 2 and 3, the bits 24 of amalgamative metal are of generally hemispheroidal or flattened configuration and are located adjacent the shoulder formed by the merger of the tubular portion 14 and the flared portion 15 of the stern S. This is the most desirable location for a watt lamp which has a T12 envelope (38.1 mm. outside diameter), an overall length of about 122 centimeters, and short stems of the type shown (approximately 3.5 centimeters long) since the amalgam which is subsequently formed within the completed lamp L operates at the proper temperature to maintain the mercury-vapor pressure within the desired range of from about 3 to 14 microns when the lamp is operated at its rated wattage.

The bits 24 of amalgamative metal are fused to the tubular portion 14 of the stem S by heating the glass sufficiently to melt or soften the metal and then simply pressing the metal bits against the heated stem and holding them in place until the glass and metal cool. Since indium and indium-rich In-Sn alloys begin to melt at around 156C. and wet glass, only a slight amount of heating is required to anchor the metal 'bits in place.

Although the fused segments 24 of metal (and the amalgam which they subsequently form with the mercury inside the finished lamp L) wet and thus inherently cling to the stem surfaces, it has been found that if the segments or bits of amalgamative metal are too large they sometimes become detached, particularly when the metal is in a liquid or semi-liquid state during lamp exhaust and the lamps are handled in the factory while they are still hot. This potential problem is avoided according to the invention by controlling the shape and size or mass of the amalgamative metal segments.

Specifically, the configuration of the metal bits 24 is such that they have sufficient surface area in contact with the surface of the glass stems ,S to insure that they will each support their own weight during lamp manufacture and subsequent handling of the lamp when the metal (or amalgam) is in a fluid condition. Care should thus be exercised to prevent the tendency of the fused metal bits to become spherical during the attachment operation and thereby reducing the contact area to a value which is too small. In addition, each piece of metal is made small enough to have sufficient cohesiveness to remain attached as a unit to the stems during lamp processing and handling in the factory. The smaller the individual segments or bits of amalgamative metal, the more favorable the adhesion and cohesion conditions become.

As a specific example, it has been found that a hemispherical piece of indium 2 millimeters in diameter has a mass of about 15.5 milligrams and a flat surface of about 3.14 square millimeters, and that bits of indium nominally 10 milligrams in mass and with a glass wetting (flat) area of approximately 3 square millimeters have satisfactory stability during the lamp-making operations and remain on the stems. Thus, in order for a 10 milligram piece of indium to have at least a 3 square millimeter contacting area, it should be flatter than hemispherical in shape. All of these criteria can be met by cutting the pieces from a flat strip or ribbon of indium and fusing them to the glass surface of the stem. As a result of the slight rounding off of the flat coinlike" pieces which occurs during the fusion process, the metal bits 24 will have a generally flat hemispheroidal shape such as that shown in FIG. 3.

In the case of a 40 watt fluorescent lamp having a length of about 122 centimeters and containing approximately 20 milligrams of mercury and about milligrams of an amalgamative metal such indium, the separation of the amalgamative metal into discrete bits each having a mass of approximately 9 milligrams requires that 10 such bits of metal be employed in a lamp of this type. Each of the stems S is thus provided with 5 circumferentially-spaeed bits 24 of amalgative metal, as shown in FIG. 1. The fact that the amalgamative material is divided into a plurality of segments affords an additional safeguard with regard to lamp operability in that if one or two of the segments should become dislodged during lamp processing or burning, the remaining segments will provide a sufficient amount of amalgam at the desired location to give proper mercury-vapor pressure control.

If pieces of amalgamative metal having a mass larger than about 10 milligrams are employed, or if bits of metal having less than 3 square mm. of contacting area are used, then supplementary means for retaining them in place on the lamp stems are required. Adequate retension can be accomplished in such cases by using a porous layer of insert material. A fluorescent lamp stem 5,, embodying this form of the invention is shown in FIG. 4. As will be noted, the bits 24a of the amalgamative metal are arranged in circumferentiaIly-spaced position around the tubular part 14a of the stem 5,, ad jacent the curved junction with the flared portion I5u as in the previous embodiment. The metal bits 24a are retained at this location by an overlying porous layer 26 of suitable material that is inert and encircles the stem in band-like fashion and permits the mercury vapor within the lamp to enter and leave the metal bits 244:. As shown, more particularly in FIG. 5, the porous layer 26 extends over and beyond each of the metal segments 24 a and is bonded tothe surrounding areas of the tubular portion 14a of the stem. The amalgamative metal pieces 24u are thus securely held in place on the stem 8,, a predetermined distance from the electrode 16a.

As a specific example, the porous retaining layer 26 can comprisea thin film of a finely-divided material, such as titanium dioxide or zirconium dioxide, that will remain stable under the conditions of lamp manufacture and operation and thus will not contaminate the lamp. A sufficiently thin and porous film can be obtained by suspending powdered titania (or zirconia, or a blend of TiO2 and ZrO2) in a nitrocellulose lacquer and applying the resultant paint over the portion of the lamp stem 8,, to which the bits 24a of amalgamative metal have been fused. Upon drying. the lacquer holds the metal bits in place while the lamp is being baked prior to and during exhaust. The elevatedtemperatures during the baking operation vaporizes the nitrocellulose and the latter is removed from the lamp, along with the volatilized material from the phosphor coating, during the exhaust operation. A thin porous film 26 of finely-divided inert material thus remains in, overlying relationship with the amalgamative metal bits 240 and adjacent parts of the stem 5,, in the finished lamp L.

When segments of amalgam metal much larger than milligrams are employed, flaking-off" of the po rous titania or zirconia coating 26 may occur as a result of the increase in the volume of the metal segments when they combine with the mercury and form the amalgam. This problem can be circumvented by including an inert binding material in the coating. As a specific example, good results have been obtained by admixing boric acid (H 80 with the powdered titania (or zirconia) and nitrocellulose vehicle. At the temperatures which occur during the sealing and exhaust operations in manufacturing the lamp, the boric acid loses H O to form boric anhydride (B 0 which fuses into a vitreous state and thus binds the titania (or zirconia) powder into an inert coating in the finished lamp that is sufficiently firm and elastic to contain the formed amalgam and yet is porous enough to permit the passage of mercury vapor into and out of the amalgamative metal. A weight ratio of around 25% boric acid in the dry mixture with an infusible inert powder (such as titania or zirconia) that is subsequently suspended in a suitable volatile vehicle, such as nitro-cellulose, has given satisfactory results. i

Other fusible binding materials may be used instead of boric oxide. For example. any of the fluorides or oxides that have relatively low melting points and low vapor pressures under lamp operating conditions can be used. Lead monoxide (PhD) is a specific example, even though its melting point (888C) would probably require that the applied coating be separately heated prior to the lamp-sealing operation in order to properly fuse the PbO. Blends of such materials can also be used.

In FIGS. 6 and 7 there is shown an alternative embodiment comprising a glass stem Sb in which the amalgamative metal is admixed with a suitable fusible binder material and a non-fusible material (such a mixture of boric acid with titania or zirconia) and formed into small pellets 28 of generally spherical shape that are less than about 1 millimeter or so in diameter and are attached to the tubular portion 14b of the stem at spaced locations adjacent the flared portion 1517. During the exhaust-bake phases of lamp manufacture, the water vapor is driven from the boric acid and forms boric anhydride which fuses with the titania (or zirconia) and provides a porous matrix thatcements or bonds the metal-containing pellets 28 to the stem S The pellets can accordingly be simply pressed onto the stem and will be automatically fused to the glass during the exhaustbaking operation. This eliminates the necessity of fusing each individual-piece of amalgamative metal to the glass, as in the case when'bits of unadulterated metal'are used.

According to a further embodiment (shown in- FIG. 8), the bits or segments 240 of amalgamative metal are pressed into recesses or indents 30 of circular or groove-like shape that are provided in the stem tube 14c and thus serve as pockets for the metal. The filling of amalgamative metal is retained in place within these pockets by a suitable vapor-porous coating 32 of inert material that extends beyond the recesses 30 and adheres to the surrounding portions of the stem S,..

As will be apparent to those skilled in the art, the recesses or pockets can be of various shapes and can also be provided on the flared portion of the stem, or on any vitreous or glass part of the lamp structure wherever an inexpensive and effective means for retaining a mercury-vapor controlling amalgam is desired. If a suitable fusible binding material is admixed with the amalgamative metal (as in the FIGS. 6 and 7 embodiment), then the vapor-porous coating 34 can be omitted. Such coating may also be omitted if the recesses in the stem are shaped to provide a constricted opening which prevents'the amalgamative metal from falling or leaking out of the recesses.

I claim as my invention:

1. In the manufacture of a glass stern assembly for a low-pressure electric discharge lamp of a type that is closed with a controlled amount of mercury after the stem assembly is sealed to the lamp envelope, the method of providing said stem assembly with integral means for controlling the mercury-vapor pressure within ,the finished lamp, said method comprising the steps of:

forming a compact body of a metal that has a melting point below the softening point of the glass stem assembly and combines with mercury to form an amalgam,

heating the glass stem assembly to a temperature above the melting point of the amalgamativemetal.

placing the body of amalgamative-metal in contact with the surface of the heated stem assembly until the metal body is fused to the stern assembly, and then maintaining the metal body and stem assembly in such position until they cool and the metal body rigidifies and is secured to the stem assembly.

2. The method of claim 1 wherein said stem assembly has a tubular portion that merges with a flared portion, and

said body of amalgamative metal is fused and secured to the tubular portion of the stem assembly adjacent the flared portion thereof.

3. The method of claim 1 wherein; the stem assembly includes an electrode that is fastened to lead wires anchored in the tubular portion of said stem assembly. and

a plurality of amalgamative metal compacts are fused and secured to the surface of the stem assembly in circumferentially-spaced position thereon a predetermined distance from said electrode.

4. The method of claim 1 wherein;

said fused body of amalgamative-metal is of such mass and configuration that it exhibits a tendency to separate from said stem assembly due to its own weight, and

a layer of material that is inert with respect to the glass stem assembly, and the dosed mercury and the other discharge-sustaining components of the finished lamp, is deposited over said fused metal body and bonded to the surrounding part of the stem assembly to retain the metal body in place on said stem assembly, said layer of inert material being porous to mercury vapor.

5. The method of claim 4 wherein said porous layer of inert material consists essentially of finely-divided titanium dioxide, finely-divided zirconium dioxide, or a mixture thereof.

6. The method of claim 1 wherein; a recess is formed in the surface of said glass assembly, and said compact body of amalgamative-metal is pressed into the recess which thus serves as a retainingpocket therefor. 7. The method of claim 6 wherein the exposed surface of said amalgamative-metal body is coated with a layer of material that (a) is inert with respect to the glass .stem assembly, and the dosed mercury and the other discharge-sustaining components of the finished lamp, (b) is porous to mercury vapor, and (c) overlies and adheres to the surrounding portion of the glass stem.

8. The method of attaching a body of amalgamativemetal to the surface of a glass stem adapted for use in a low-pressure electric discharge lamp, which method comprises;

admixing the amalgamative-metal with amounts of a fusible binder and a non-fusible material sufficient to form a fused porous matrix when heated to a predetermined temperature, shaping the resulting mixture into a pellet-like body and temporarily attaching the latter to the surface of the stem by pressing it thereagainst, and then heating the stem and pellet-like body to said predetermined temperature to form said fused porous matrix and thereby permanently attach the pellet to the stem.

9. The method of claim 8 wherein said fusible binder comprises boric acid and said non-fusible material comprises titania or zirconia. 

1. In the manufacture of a glass stem assembly for a lowpressure electric discharge lamp of a type that is dosed with a controlled amount of mercury after the stem assembly is sealed to the lamp envelope, the method of providing said stem assembly with integral means for controlling the mercury-vapor pressure within the finished lamp, said method comprising the steps of: forming a compact body of a metal that has a melting point below the softening point of the glass stem assembly and combines with mercury to form an amalgam, heating the glass stem assembly to a temperature above the melting point of the amalgamative-metal, placing the body of amalgamative-metal in contact with the surface of the heated stem assembly until the metal body is fused to the stem assembly, and then maintaining the metal body and stem assembly in such position until they cool and the metal body rigidifies and is secured to the stem assembly.
 2. The method of claim 1 wherein said stem assembly has a tubular portion that merges with a flared portion, and said body of amalgamative metal is fused and secured to the tubular portion of the stem assembly adjacent the flared portion thereof.
 3. The method of claim 1 wherein; the stem assembly includes an electrode that is fastened to lead wires anchored in the tubular portion of said stem assembly, and a plurality of amalgamative metal compacts are fused and secured to the surface of the stem assembly in circumferentially-spaced position thereon a predetermined distance from said electrode.
 4. The method of claim 1 wherein; said fused body of amalgamative-metal is of such mass and configuration that it exhibits a tendency to separate from said stem assembly due to its own weight, and a layer of material that is inert with respect to the glass stem assembly, and the dosed mercury and the other discharge-sustaining components of the finished lamp, is deposited over said fused metal body and bonded to the surrounding part of the stem assembly to retain the metal body in place on said stem assembly, said layer of inert material being porous to mercury vapor.
 5. The method of claim 4 wherein said porous layer of inert material consists essentially of finely-divided titanium dioxide, finely-divided zirconium dioxide, or a mixture thereof.
 6. The method of claim 1 wherein; a recess is formed in the surface of said glass assembly, and said compact body of amalgamative-metal is pressed into the recess which thus serves as a retaining-pocket therefor.
 7. The method of claim 6 wherein the exposed surface of said amalgamative-metal body is coated with a layer of material that (a) is inert with respect to the glass stem assembly, and the dosed mercury and the other discharge-sustaining components of the finished lamp, (b) is porous to mercury vapor, and (c) overlies and adheres to the surrounding portion of the glass stem.
 8. The method of attaching a body of amalgamative-metal to the surface of a glass stem adapted for use in a low-pressure electric discharge lamp, which method comprises; admixing the amalgamative-metal with amounts of a fusible binder and a non-fusible material sufficient to form a fused porous matrix when heated to a predetermined temperature, shaping the resulting mixture into a pellet-like body and temporarily attaching the latter to the surface of the stem by pressing it thereagainSt, and then heating the stem and pellet-like body to said predetermined temperature to form said fused porous matrix and thereby permanently attach the pellet to the stem.
 9. The method of claim 8 wherein said fusible binder comprises boric acid and said non-fusible material comprises titania or zirconia. 